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Abstract

Fischer-Tropsch Reaction Water (FTRW) is a high organic strength wastewater produced as a by-product in
Sasol’s Fischer-Tropsch Reactors. Typically it has an organic load of 18000 mgCOD/L and is highly acidic
with a pH of approximately 3.8. It is deficient in nutrients (N and P and other micronutrients).
This dissertation deals with the biological and physico-chemical model development of a dynamic anaerobic
digestion model, and explores two different approaches to representing the physico-chemical processes that
complement and interact with the bioprocesses. The performances of the resultant two dynamic models (ADFTRW1
& AD-FTRW2) were compared in order to assess to what extent the more detailed and rigorous
ionic speciation modeling in AD-FTRW2 addressed the shortcomings attributed to the simplified physicochemical
modeling in AD-FTRW1.
The ionic speciation model used in AD-FTRW2 uses a classic equilibrium formulation along the same lines
as in the UCTADM2 model for anaerobic digestion of municipal wastewater sludges (Brouckaert et al.,
2010), while AD-FTRW1 uses a simplification of the approach developed by Musvoto et al. (2000) in order
to represent short chain fatty acid (SCFA) dissociation and the weak acid base chemistry of the inorganic
carbon system.
A 44 day extract from a 700 day laboratory-scale dataset (Van Zyl et al. 2008) was used as the basis for
comparing the models. During this period the membrane bio-reactor was subjected to varying flow and load
conditions. To validate the models, the experimentally measured and model predicted process variables of
reactor alkalinity, reactor pH, biogas production and effluent SCFA concentration were compared.
It was found that AD-FTRW2 provided superior agreement with pH data, but predictions of alkalinity, gas
production rate and effluent short-chain fatty acids were not significantly improved in AD-FTRW2 relative
to AD-FTRW1. This outcome was hypothesized since pH is strongly dependent on physico-chemical
processes such as ionic interactions in solution and gas exchange which were the components to the models
(AD-FTRW1 versus AD-FTRW2) which differed most significantly. Alkalinity, which is also highly
influenced by physico-chemical model representations showed substantial improvement however statistical
analysis could not show this improvement to be significant. The other two variables that were compared,
biogas production and effluent SCFA concentration, displayed very similar agreement with experimental
data. These variables depend more on mass balance effects and biological kinetics and were therefore not
significantly altered by the more rigorous handling of aqueous chemistry in AD-FTRW2. It was concluded
that AD-FTRW2 constitutes an improvement in model predictive power over AD-FTRW1 at a small cost in
computing time.